Submitted to: Mycological Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/20/2005
Publication Date: 11/1/2005
Citation: Bae, H., Bowers, J.H., Tooley, P.W., Bailey, B.A. 2005. Nep1 orthologs encoding the necrosis and ethylene inducing proteins exist as a multigene family in phytophthora megakarya, causal agent of black pod disease on cacao. Mycological Research. 109:1373-1385. Interpretive Summary: The plant pathogen, Phytophthora megakarya causes black pod disease in the tropical tree Theobroma cacao (cacao) and the disease severely reduces cacao yields around the world. Phytophthora species produce proteins that kill plant cells and are thought to be important in plant susceptibility to disease. We have cloned and characterized the Phytophthora megakarya genes that produce one of these proteins that kill cacao cells. Multiple copies of the genes producing the plant cell killing proteins were identified in P. megakarya, as well as in four other Phytophthora species. By understanding how black pod disease develops, it may be possible to develop cacao plants resistant to disease and/or develop other disease control methods, which will benefit farmers and industries there by improving cacao yields.
Technical Abstract: Phytophthora megakarya is a devastating fungal pathogen that causes black pod disease in cacao. Phytophthora species produce a protein that has a similar sequence to the necrosis and ethylene inducing protein (Nep1) of Fusarium oxysporum. Multiple copies of NEP1 orthologs (PmegNEP) have been identified in P. megakarya strain Mk-1 and four other Phytophthora species. Genome database searches confirmed the existence of multiple copies of NEP1 orthologs in P. sojae and P. ramorum. In this study, nine different PmegNEP orthologs from P. megakarya strain Mk-1 were identified and analyzed. Of these nine orthologs, six were expressed in mycelium and in zoospore-infected cacao leaf tissue. Sequence analysis revealed that six PmegNEP orthologs were organized in two clusters of three orthologs each in the P. megakarya genome. Evidence is presented for instability in the P. megakarya genome resulting from duplications, inversions, and fused genes resulting in multiple NEP1 orthologs. Traits characteristic of the Phytophthora genome, such as the clustering of NEP1 orthologs, the lack of CATT and TATA boxes, the lack of introns, and the short distance between ORFs were also observed. The structure and regulation of NEP1 orthologs in Phytophthora species are much more complex than previously thought. The multiple and ubiquitous existence of this gene family in Phytophthora species as well as many other organisms leads us to suggest that the gene has an important function in microbial biology.